I'm talking about screenshots like these:
These images appear completely non-pixelated and in high resolution. The first one is a a vanilla, operating-system level menu bar in Mac OS, and the second one is in Blender, which does all its own menus.
Is there a trick to getting screenshots like this?
Those screenshots are probably made on a retina screen since the UI is also in high resolution. Normally when you use a magnifier tool you lose quality because of the zooming.